1. Field of the Invention
The present invention relates to an image reading lens for reading an original placed on an original table and an image reading apparatus using the image reading lens. In particular, the present invention is suitable for image reading apparatus such as an image scanner, a copying machine, and a facsimile machine, which are capable of high-precision image reading by fully utilizing the optical performance of an image reading lens having an anamorphic lens.
2. Description of the Related Art
Up to now, as apparatus for reading image information of an original placed on an original table, image reading apparatus such as an image scanner and a digital copying machine are known. Of the image reading apparatus, there is known a flatbed type image scanner using an integrated scanning optical unit (carriage-integrated unit) in which an imaging optical system for image reading and a reading unit are contained in a case (carriage).
In this image reading apparatus, the original placed on the original table is illuminated directly or through a reflection shade with a light beam emitted from an illumination light source. An optical path of the light beam reflected on the original is bent in an inner portion of the carriage through multiple reflection mirrors. The image information of the original is imaged by the imaging optical system on a surface of a linear image sensor (hereinafter referred to as “CCD sensor” or “CMOS sensor”) in which reading pixels are arranged in a main scanning direction. The carriage is moved in a sub scanning direction by a sub scanning motor to read the image information of the original. The read image information is sent to an external device, for example, a personal computer through an interface.
In recent years, the image reading apparatus having the structure described above is desired to be reduced in size. In order to achieve the reduction in size, it is important to reduce the size of the carriage. In order to reduce the size of the carriage, for example, there are a method of increasing the number of reflection mirrors and a method of performing multiple reflections using a single reflection mirror to ensure an optical path length. In addition to this, in recent years, there is proposed an image reading apparatus in which an anamorphic lens having at least one surface with a rotationally asymmetric shape about an optical axis is introduced into the imaging optical system (image reading lens). In the case of the image reading apparatus, a view angle of the imaging optical system (imaging system) is widened to reduce an object-to-image distance, to thereby shorten the optical path length and reduce the overall size of the apparatus.
When the anamorphic lens is used for the imaging optical system as described above, an image field curvature aberration may be effectively reduced, and hence image information with an excellent contrast may be obtained. However, when the anamorphic lens is used, an imaging magnification, particularly, an imaging magnification in the sub scanning direction (sub-scanning magnification) may be changed from the optical axis to a peripheral region. Therefore, when an image is to be read by a three-line color sensors of R, G, and B of a color image reading apparatus, an imaging position is shifted for each of the colors, and color drift may occur in the sub scanning direction.
In contrast to this, there is proposed a color image reading apparatus in which the specifications (shapes) of a line sensor and a distortion component of an image reading lens in a sub scanning direction are suitably set to obtain an excellent color image (see Japanese Patent Application Laid-Open No. 2000-307800). Further, there is proposed a color image reading apparatus in which a shape of a lens having two anamorphic surfaces is suitably set to obtain an excellent color image (see Japanese Patent Application Laid-Open No. 2008-065234).
In recent years, there is a strong demand for the image reading apparatus to read image information with high contrast and high precision during the reading of image information. In order to read the image information with high precision in the image reading apparatus, it is important to reduce sagittal flare and an image field curvature in the main scanning direction. If the sagittal flare and the image field curvature are not suitably corrected, it is difficult to excellently read the image information with high contrast over the entire region in the main scanning direction.